Software-how do we know it is safe?

Microprocessors are used in nearly every system of a rail vehicle from the brake controls to the automatic train control. Microprocessors are being used in systems that have been traditionally designed using fail-safe hardware techniques proven safe over the years. The use of microprocessors to replace these traditional systems raises a new set of concerns and methods to verify their safe operation. This paper concentrates on the safety related aspects of software as a basic primer on software safety giving some of the concerns and methods to mitigate these concerns.